1. Field of the Invention
The present invention is directed to a RIM process using a molding composition which contains an internal mold release agent mixture based on a zinc carboxylate and a compatibilizer.
2. Description of the Prior Art
Reaction injection molding (RIM) has become an important process for the fabrication for external automotive body parts and other industrial thermoset moldings. The RIM process is a so-called "one-shot" process which entails the intimate mixing of a polyisocyanate component and an isocyanate-reactive component followed by the injection of this admixture into a mold for subsequent rapid curing. The polyisocyanate component is generally based on a liquid polyisocyanate. The isocyanate-reactive component contains a high molecular weight isocyanate-reactive component, usually a polyol, and optionally a chain extender containing amino or hydroxyl groups. U.S. Pat. No. 4,269,945 discloses a specific chain extender which contains an aliphatic amine-containing compound having at least one primary amino group such as an aminated polyoxypropylene glycol, and either a hydroxyl-containing compound free of aliphatic amine hydrogen atoms or an aromatic amine hydrogen-containing compound which is free of aliphatic amine. U.S. Pat. No. 4,218,543 discloses a chain extender which is miscible with the polyol component and is based on compounds containing sterically hindered amino groups such as 1-methyl-3,5-diethyl-2,4-diamino benzene, 1-methyl-3,5-diethyl-2,6-diamino benzene or mixtures thereof.
While products produced by the RIM process have excellent physical properties, the use of the RIM process has been limited by the necessity to spray external mold release agents (waxes, soaps, etc.) onto the mold surface before each shot or every several shots. Molding without such an external release agent results, using standard molding compositions, in parts that are torn, distorted or otherwise damaged because of the adhesion of the molded part to the mold surface. Several attempts have been made to formulate reaction components which contain an internal mold release agent so that the need to repeatedly apply external mold release agents to the mold surface would either be eliminated or diminished.
U.S. Pat. No. 4,379,100 is directed to the use of a polycarboxy-functional alkyl siloxane as an internal release agent; however, this internal release agent is not preblended with either the polyisocyanate or polyol component, but rather is introduced as a third stream to the mixhead of the injection apparatus. U.S. Pat. No. 4,076,695 is also directed to the use of a carboxyfunctional siloxane which may be incorporated with either a polyisocyanate or polyol component as an internal mold release agent. A third patent directed to the use of carboxy-functional silanes is U.S. Pat. No. 4,396,729. However, it is disclosed therein that amino-functional reactants are necessary since polyol reactants require the use of a tin catalyst which interacts with the carboxy groups of the release agent resulting in inferior products. U.S. Pat. No. 4,374,222 is directed to the use of hydroxyl-substituted fatty acid amides as an internal mold release agent for use in the RIM process.
The use of metallic carboxylic acid salts as an internal mold release agent for various systems has also been disclosed. For example, U.S. Pat. No. 4,067,845 is directed to molded unsaturated polyester resin systems utilizing a dual thickening system based on a metallic oxide and a polyisocyanate and, in addition, a mold release agent based on zinc stearate. British Patent No. 2,101,140 is directed to an internal mold release agent based on a metallic carboxylate salt such as zinc stearate and an epoxidized vegetable oil such as epoxidized soybean oil for use in a RIM process.
The use of zinc stearate as a catalyst is disclosed in U.S. Pat. No. 4,119,594, wherein a mixture of solid particles of hydroquinone di-(beta-hydroxyethyl)-ether (as chain extender) and zinc stearate (as catalyst) are dispersed in a liquid mixture of a polyether polyol and a liquid form of diphenylmethane diisocyanate and subsequently reacted in a RIM process to produce thermoset polyurethanes.
British Patent No. 1,476,997 discloses the use of metallic carboxylates as surface quality improvers for polyamides, while U.S. Pat. No. 3,705,860 discloses the use of metallic carboxylates as antifoam agents for paints or paper pulp compositions.
One object of the present invention is to provide RIM molding compositions which would eliminate or decrease the frequency of application of an external mold release agent to the surface of a mold, thereby, providing higher productivity. A further object of the invention is to provide such molding compositions without introducing detrimental effects on the physical properties (e.g., heat sag, flexural modulus, tear strength or tensile strength), processing characteristics (e.g., demold time, toughness at demold known as "green strength" or flowability), or post-molding operations, especially paintability.
It has been discovered that the objects of this invention can be realized by the modification of RIM molding compositions with the internal mold release agent mixtures hereinafter described.